Summary: Articles
Theoretical Study of Ladder Polysilanes
Monica Kosa, Miriam Karni,* and Yitzhak Apeloig*
The Schulich Faculty of Chemistry and the Lise Meitner-MinerVa Center for Computational Quantum
Chemistry, Technion-Israel Institute of Technology, Haifa 32000, Israel
ReceiVed NoVember 9, 2006
The parent ladder polysilanes R4Si2-(Si2R2)n-Si2R4 (R ) H, n ) 0-7), composed of a catenation of
cyclotetrasilane rings, and their corresponding methyl- (R ) CH3) and silyl-substituted (R ) SiH3)
derivatives were studied computationally using quantum mechanical methods, mainly density functional
theory. The calculations show that the parent ladder polysilanes have puckered four-membered rings for
n ) 0-3, 5 and nearly planar rings for n ) 4, 6, 7 and, consequently, the parent ladder polysilanes are
not twisted. In contrast, methyl- and silyl-substituted ladder polysilanes have twisted four-membered
rings and the ladder is helical. This is consistent with experimental data for R ) iPr, which show twisting
at each tetrasilacyclobutane ring, leading to a helical ladderane structure. The electronic properties of
ladder polysilanes are similar to those of oligosilanes with the same number of Si-Si bonds. Upon
chain elongation, the HOMO-LUMO gap decreases, reaching a gap of ca. 3.4 eV for n ) 7 and ca.
2.3-2.7 eV, approaching the semiconductor range, when extrapolated to the polymeric limit, significantly
smaller than the gap of 4.7 eV obtained computationally for regular polysilanes. These trends found
computationally are consistent with the experimental UV-vis lowest energy transitions, which show a
shift to longer wavelengths as the ladder is elongated.